A typical web conveyance drive in a web section where contact is possible with only one face of the web consists of a plurality of adjacent, parallel in-line drive rollers nested in a suction box with the web held to the rollers by reduced air pressure created in the suction box. In one prior known web conveyance drive of this type, the rollers have been driven and synchronized together by chains or belts from a single motor. While this arrangement assures that all rollers in the drive rotate at the same angular speed, it has several operational problems. The belts or chains have a tendency to generate dirt which can contaminate the web product. They are also subject to wear and mechanical failure and are a source of machine failure from time to time. Moreover, assuring that all of the rollers rotate at the same angular speed does not assure that the rollers will have the same surface speed required for proper operation of the web drive. In order to assure adequate control over roller surface speed, it is necessary to precision machine special rollers to very tight diameter specifications. This requires special manufacturing and stocking procedures and prohibits the use of refinished rollers.
A known alternative to chain or belt synchronized drives is to drive the rollers individually, one motor per roller, with the rotation of the rollers held together by "electronic synchronization" provided by AC motors driven by a variable-frequency drive or servo motors held to relative position by position feedback. The controlled AC motors drive arrangement provides adequate load sharing between the motors but results in speed control with inadequate frequency response. Such response typically results in stability difficulties in tuning the tension/position loop associated with the drive, which means the tension/position loops gains are too low to adequately control web tension or the amount of web in the controlled web span.
Although the alternative of using positioning servo motor drives is effective to lock the angular position of each roller to the others, it requires the same precise control over roller diameters as in the belt/chain synchronizing approach. It is, of course, possible to compensate for roller diameter variations through fine electronic adjustment of the ratios of the speed of the various rollers, however, such an arrangement requires complicated and expensive control circuitry and sensing equipment (encoders, pulse rate multipliers, etc.) that add substantially to the cost of the drive system. It also requires that the controls be carefully recalibrated every time a roller is replaced.
Another known approach to web drive roller synchronization involves "current compounding" of the drive motor currents. In this approach, a signal proportional to motor current is subtracted from the speed reference applied to each drive motor which causes the motors with the highest currents to have the most speed subtracted from their speed references. In practice however, this approach has been found to be impractical because loop gains sufficient to actually level motor currents in the face of typical tachometer errors cause control system instability. Another disadvantage is that an increased load on the suction box drive (for example, higher web tension at the suction box inlet) causes a net decrease in the actual speed of the box. This speed change would require compensation by an external position loop controller. It also would prevent the use of such a suction box as a machine master for the conveyor system, since its speed would vary with load.
It is therefore an object of the present invention to provide a web conveyor drive system of the type described that maintains roller speed synchronization without the use of belts or chains and without the use of complicated and costly electronically controlled servo positioning drives.
It is a further object of the invention to provide such a web conveyor drive that maintains substantially equal surface speeds among the rollers despite significant differences in roller diameters thus allowing the use of less costly rollers and facilitating the maintenance considerations.
It is a still further object of the invention to provide such a web conveyor that maintains a relatively constant web speed in the face of variations in input tension thus allowing use of the web drive as a machine master drive system.